feat: added attr-no-unnecessary-whitespace rule

[jakewtaylor]

Shows an error like:
The attribute 'name' must not have spaces between the name and value.
for code like:

type = "text"
type ="text"
type= "text"

This is valid:

type="text"

[jakewtaylor]

I've also added support for PHP files.

Anything between <?php/<? and ?> tags is stripped out and replaced with blank text with the same amount of newlines as before, so that errors still have the same line numbers.

I've added a test to check that PHP is properly ignored.

[jakewtaylor]

Not really sure how to fix the coverage check failure - let me know how to get that to pass and I'll fix.

[Shinigami92]

Since lib/htmlhint.js is irrelevant to the current development phase, I think it is safe to merge this and bring it into the 'develop' branch

[thedaviddias]

Closing this old PR, reopened in #385 to speed up fixing merge conflicts.